Friction material such as that used in brake lining, clutches, and the like has severe performance requirements. The principal function of a friction element like a brake lining is to convert kinetic energy to heat and to absorb the heat or otherwise dissipate it while simultaneously through the agency of friction to reduce relative movement between the friction material and a part engaged by it. To achieve these objectives, it is necessary that the coefficient of friction between the friction material and the part so engaged be as high as possible, independent of variations in operating conditions, and accomplish the necessary energy conversion with a minimum wear of contacting parts. In particular, a friction material should not only have a relatively high coefficient of friction, but as well possess durability, heat stability, generate little or no noise while in rubbing contact with an engaging part, such as a rotor, and cause a minimum of wear on the engaged part.
In general, a friction material contains a matrix or binder, such as a thermosetting resin or vulcanized rubber, a fibrous reinforcement, and a friction modifier which aids in imparting a desired coefficient of friction to the material. Often the friction material may also contain fillers or extenders which modify its physical characteristics and reduce its cost. The fibrous reinforcement is usually asbestos, although fibers of other high temperature resistant materials can be used as well to hold or bind together the components of friction material.
Under certain conditions such as overheating or contacting water, asbestos fibers have a tendency to glaze so that a lower coefficient of friction results. Subsquently, when an operator of a car, for example, attempts to operate the brake, the ability of the brake lining to perform as designed is seriously hampered and accidents have occurred as a result. Accordingly, an additive friction modifier which increased the coefficient of friction would be especially useful in friction material containing asbestos as the fibrous reinforcement.
Many frictional elements like brake linings have a tendency toward "fade," which may be considered to be the inability to maintain a reasonably constant value of friction during repeated application of a brake system at a given speed, usually due to overheated break linings. All linings exhibit fade to some extent. The inclusion of heat conducting particles, such as metal particles, in the friction material has been suggested to reduce the tendency to fade. However, this shortcoming still remains a serious problem.